The present invention relates generally to a magneto-optical disk, and more particularly to a magneto-optical disk having at least a groove on the surface of a substrate and a magneto-optical recording method for the magneto-optical disk.
A magneto-optical disk is built up of a substrate having a groove and pits, on which a magneto-optical recording layer is formed. At the time of recording and reproduction the intensity of light reflected from the vicinity of the groove is detected to do tracking control, track counting during seeking, etc.
Among properties taking part in these controls there are a push-pull signal level, a radial contrast, and a skew margin.
The push-pull signal is understood to refer to a tracking signal generated when tracking is controlled by the push-pull method. Here it is noted that the pull-push method is one in which light reflected and diffracted by the groove or pits is received by two light receiving portions of a split-half photodiode that are disposed symmetrically with respect to the center of the track to detect a tracking error on the basis of a difference in the resulting outputs. If I.sub.1 and I.sub.2 represent the outputs of the light-receiving portions, respectively, the push-pull signal level P-P will be given by EQU (I.sub.1 -I.sub.2)/(I.sub.1 +I.sub.2)
At too low a push-pull signal level normal tracking is unlikely to occur. When the push-pull signal level is too high, on the other hand, it is incompatible with other optical properties and, although depending on the type of optical head, noises are likely to occur in the focus-servo signal. Generally, it is desired that the push-pull signal level be within the ranges of 0.11 to 0.20 and 0.04 to 0.11 in the groove and pits, respectively.
For instance, now let I.sub.L and I.sub.G designate the outputs of the land and groove portions of a signal in a grooved area, which is generated when a low-pass filter is used. Then the radial contrast RC is given by EQU RC=2.vertline.I.sub.L -I.sub.G .vertline./(I.sub.L +I.sub.G)
From the RC output it is possible to know the number of tracks which are jumped over by the optical head and the direction of movement (polarity) of the optical head. When the radial contrast is too indefinite, an error is likely to occur in counting tracks or judging polarity, whereas when the radial contrast is too sharp, disturbance noises are likely to make the servo system unstable. It is generally desired that the radial contrast RC be within the range of 0.20 to 0.35 in the groove and within the range of 0.15 to 0.30 in the pit, respectively.
The term "skew margin" used in the present disclosure is understood to mean to what degree information is read when a magneto-optical disk inclines. For instance, consider the case where the disk inclines to an associated optical pickup in the radial direction. As the angle of inclination increases, more increased are errors in reading signals. Eventually the errors exceed the demanded (standard) value. In other words, the skew margin is understood to refer to the angle range in which signals are read within the standard value in a stable manner. The larger this angle, the better.
The push-pull signal level, radial contrast and skew margin vary depending on groove width and depth, and on the basic angle or bottom angle of the groove-forming side walls as well. Here, too, C/N varies. In other words, the radial contrast increases with an increase in groove depth (provided that groove depth.ltoreq..lambda./4n). This is also true even when the basic angle .theta. is increased so as to make the groove wide. The push-pull signal level decreases with a decrease in groove depth (provided that groove depth.ltoreq..lambda./8n) and, again, with an increase in groove width (provided that groove pitch=1.6 .mu.m and groove width.gtoreq.0.8 .mu.m). C/N increases with an increase in groove width or with an increase in the basic angle .theta.. The skew margin decreases with an increase in the basic angle .theta.. Therefore, much difficulty is involved in allowing all the push-pull signal level, radial contrast and skew margin to have satisfactory values and obtain good-enough C/N as well. Here it is desired that C/N be 47 dB or more.
It is therefore a primary object of the present invention to provide a magneto-optical disk having satisfactory values for its push-pull signal level, radial contrast and skew margin, and having good-enough C/N as well.